Structure-activity relationship studies of melanin-concentrating hormone (MCH)-related peptide ligands at SLC-1, the human MCH receptor

Melanin-concentrating hormone (MCH) is a cyclic nonadecapeptide involved in the regulation of feeding behavior, which acts through a G protein coupled receptor (SLC-1) inhibiting adenylcyclase activity. In this study, 57 anal ogues of MCH were investigated on the recently cloned human MCH receptor st ably expressed in HEK293 cells, on both the inhibition of forskolin-stimula ted cAMP production and guanosine-5'-O-(3-[S-35]thiotriphosphate ([S-35]GTP gammaS) binding. The dodecapeptide MCH-(6-17) (MCH ring between Cys(7) and Cys(16), with a single extra amino acid at the N terminus (Arg(6)) and at the C terminus (Trp(17))) was found to be the minimal sequence required for a full and potent agonistic response on cAMP formation and [S-35]GTP gamma S binding. We Ala-scanned this dodecapeptide and found that only 3 of 8 ami no acids of the ring, namely Met(8), Arg(11), and Tyr(13), were essential t o elicit full and potent responses in both tests. Deletions inside the ring led either to inactivity or to poor antagonists with potencies in the micr omolar range. Cys(7), and Cys(16) were substituted by Asp and Lys or one of their analogues, in an attempt to replace the disulfide bridge by an amide bond. However, those modifications were deleterious for agonistic activity . In [S-35]GTP gammaS binding, these compounds behaved as weak antagonists (K-B 1-4 muM). Finally, substitution in MCH-(6-17) of 6 out of 12 amino aci ds by non-natural residues and concomitant replacement of the disulfide bon d by an amide bond led to three compounds with potent antagonistic properti es (K-B = 0.1-0.2 muM). Exploitation of these structure-activity relationsh ips should open the way to the design of short and stable MCH peptide antag onists.